Radio wave absorbing panel

ABSTRACT

A radio wave absorbing panel is made up of a first insulating substrate and a second insulating substrate disposed in parallel a predetermined distance apart and a middle insulating substrate disposed between and parallel with the first and second substrates. The first and second substrates each have a conducting film coated over the entire surface of one side. On one side of the middle insulating substrate are coated multiple conducting films disposed in the form of stripes or a matrix. By this means a thin radio wave absorbing panel having excellent radio wave absorption and transparency to light can be obtained.

TECHNICAL FIELD

The present invention relates generally to a radio wave absorbing panelused for absorbing radio waves incident on a building and, moreparticularly, to a radio wave absorbing panel which both absorbsincident radio waves and transmits visible light.

BACKGROUND ART

In recent years, along with increases in numbers of high-rise buildings,cases of radio waves of TV frequency bands such as VHF and UHF beingreflected by buildings have become more common. Consequently, ghosting,which arises on a TV screen when radio waves arriving at the antenna ofa TV receiver directly from a TV station (direct waves) and radio wavesreflected by buildings (reflected waves) are incident on the antennasimultaneously, have become a serious social problem. For this reason,with the object of reducing the number of these radio waves reflected byexterior walls of buildings, radio wave absorbing panels made ofmagnetic materials such as ferrite have been affixed to or embedded inexterior walls to absorb radio waves incident on buildings.

For window glass installed in building window openings, to reduceair-conditioning cooling loads in summer (to save energy), glass coatedwith a film having a heat ray reflecting function has been used;however, because films having a high heat ray reflecting capability havelow electrical resistance, their reflectivity of radio waves is high andthey are a cause of radio wave obstruction.

Radio wave absorbing panels for reducing radio wave reflection in whichferrite is used cannot be applied to window openings of building becauseferrite does not transmit visible light. Consequently the situation hasbeen that heat ray reflecting ability has been sacrificed andtransparent heat ray reflecting films having relatively high electricalresistance have been coated on windows of buildings to reduce radio wavereflection and prevent radio wave obstruction by transmitting radiowaves through window openings into buildings.

Concerning window glass for buildings and vehicles, technology is known(from, for example, Japanese Patent Laid-Open Publication Nos. HEI3-250797, HEI 5-042623, HEI 5-050548 and HEI 7-242441), whereby, withthe object of preventing obstruction due to radio wave reflection ofhigh-performance heat ray reflecting films, high heat ray reflectivityand low radio wave reflectivity are realized at the same time by aconducting film being divided up into areas of a size amply smaller thanthe wavelength of incident radio waves to raise its radio wavetransmittivity.

Radio wave absorbing panels in the related art which have radio wavetransmittivity are an attempt to prevent radio wave reflection problemsby providing window glass of buildings with radio wave transmittivity;however, associated with these there are the problems that incomingradio waves penetrate to the inside of the building and affect officeequipment such as personal computers and that electromagnetic wavesradiated from electronic appliances inside the building leak through thewindow glass to outside the building. Although this kind of radio waveobstruction is expected to increase in the future, no effectivecountermeasure has been taken besides reflecting and thereby blockingradio waves by using a conducting wire mesh or a conducting film onwindows of buildings, and in districts where there is a likelihood of aradio wave reflection obstruction arising it has been difficult to buildbuildings which have large-area windows and block radio waves.

To solve this problem, it is necessary to create a practically usabletransparent panel which absorbs radio waves instead of reflecting ortransmitting them.

At present there are radio wave absorbing panels made by disposing inparallel two transparent substrates each coated with a conducting filmhaving a controlled sheet resistivity, with which panels it is possibleto realize a high radio wave absorbing capacity by utilizing resonancecaused by interference of multiple reflections of radio waves. However,to absorb VHF band (about 100 MHz) radio waves, the gap between the twosubstrates constituting the radio wave absorbing panel must be made fromseveral tens of cm to over a meter, and therefore such panels cannot berealistically applied to ordinary windows of buildings.

DISCLOSURE OF THE INVENTION

It is therefore an object of the present invention to provide a radiowave absorbing panel having superior radio wave absorbency andtransparency to light.

To achieve this and other objects, the invention provides a radio waveabsorbing panel comprising at least two insulating substrates disposedin parallel a predetermined distance apart on at least one side of eachof which is formed a continuous conducting film and at least oneinsulating substrate disposed between these insulating substrates inparallel therewith on a surface of which are formed conducting filmsdisposed in the form of stripes or in the form of a matrix.

As a result, in the invention, even when the relative permittivity ofthe radio wave absorbing panel is made large and the thickness of thepanel is made thin it is possible to make its absorptance of radio waveshigh, and consequently the invention can be applied for example to apanel for absorbing VHF band radio waves highly suitable forinstallation in a window opening of a building.

In a radio wave absorbing panel according to the invention, whenstripe-form conducting films are used, preferably, the sheet resistivityof the continuous conducting film is from 1Ω/□ to 3000Ω/□ and when thewidth and the sheet resistivity of each of the conducting films disposedin the form of stripes are respectively written Hcm and R_(BM)Ω/□ andthe insulation resistance of the insulating substrate on which thestripe-form conducting films are formed is written R_(D)cmΩ, then H,R_(BM) and R_(D) are set in the ranges of: 1 cm≦H≦50 cm,1Ω/□≦R_(BM)≦40Ω/□, R_(D)≧30,000 cmΩ. When stripe-form conducting filmshaving these values are used, the relative permittivity of the radiowave absorbing panel can be made large and its absorbtance of radiowaves raised, and even if the panel is made thin it is possible toimprove its absorbency of radio waves arriving from a fixed direction.

When matrix-form conducting films are used, preferably, the sheetresistivity of the continuous conducting film is from 1Ω/□ to 3000Ω/□and when the width, the length and the sheet resistivity of each of theconducting films disposed in the form of a matrix are respectivelywritten Hcm, Vcm and R_(BM)Ω/□ and the insulation resistance of theinsulating substrate on which the matrix-form conducting films areformed is written R_(D)cmΩ, then H, V, R_(BM) and R_(D) are set in theranges of: 1 cm≦H≦50 cm, 1 cm≦V≦50 cm, 1Ω/□≦R_(BM)≦40Ω/□. Whenmatrix-form conducting films having these values are used, the relativepermittivity of the radio wave absorbing panel can be made large and itsabsorptance of radio waves raised, and even if the panel is made thin itis possible to improve its absorbency of radio waves arriving from anydirection.

Also, when stripe-form conducting films are used, preferably, the sheetresistivity of the continuous conducting film formed on the surface ofone of the insulating substrates is not more than 30Ω/□ and the sheetresistivity of the continuous conducting film formed on the surface ofanother of the insulating substrates is from 50Ω/□ to 3000Ω/□ and whenthe width and the sheet resistivity of each of the conducting filmsdisposed in the form of stripes are respectively written Hcm andR_(BM)Ω/□ and the insulation resistance of the insulating substrate onwhich the conducting films disposed in the form of stripes are formed iswritten R_(D)cmΩ, then H, R_(BM) and R_(D) are made: 1 cm≦H≦50 cm,1Ω/□≦R_(BM)≦40Ω/□, R_(D)≧30,000 cmΩ. By making the sheet resistivity ofthe conducting film on one of the insulating substrates not more than301Ω/□ and the sheet resistivity of the conducting film of another ofthe insulating substrates from 50Ω/□ to 3000Ω/□ in this way, it ispossible to make the panel still thinner while maintaining an ampleradio wave absorbency.

And when matrix-form conducting films are used, preferably, the sheetresistivity of the continuous conducting film formed on the surface ofone of the insulating substrates is not more than 30Ω/□ and the sheetresistivity of the continuous conducting film formed on the surface ofanother of the insulating substrates is from 50Ω/□ to 3000Ω/□ and whenthe width, the length and the sheet resistivity of each of theconducting films disposed in the form of a matrix are respectivelywritten Hcm, Vcm and R_(BM)Ω/□ and the insulation resistance of theinsulating substrate on which the conducting films disposed in the formof a matrix are formed is written R_(D)cmΩ, then H, V, R_(BM) and R_(D)are set to: 1 cm≦H≦50 cm, 1 cm≦V≦50 cm, 1Ω/□≦R_(BM)≦40Ω/□, R_(D)≧30,000cmΩ. By making the sheet resistivity of the conducting film on one ofthe insulating substrates not more than 30Ω/□ and the sheet resistivityof the conducting film of another of the insulating substrates from50Ω/□ to 3000Ω/□ in this way, and because matrix-form conducting filmsset to predetermined values are being used, it is possible to make therelative permittivity of the radio wave absorbing panel large and raiseits radio wave absorptance.

Preferably, transparent plate glass is sued as the insulating substratesand the stripe or matrix-form conducting films are transparentconducting films composed mainly of SnO₂ or In₂O₃ or are metal filmscomposed mainly of Ag, Au, Cu or Al, whereby it is possible to lower thesheet resistivity of the conducting films and raise their transmittivityof light.

Also, dry air may be preferably sealed in spaces between the at leasttwo insulating substrates and the at least one insulating substrate onthe surface of which are formed conducting films disposed in the form ofstripes or a matrix, whereby condensation due to changes in outsidetemperature can be prevented and deterioration in radio wave absorbingcapacity due to water in the conducting films can be prevented.

Alternatively, resin may be preferably sealed in spaces between the atleast two insulating substrates and the insulating substrate on thesurface of which are formed conducting films disposed in the form ofstripes or a matrix to form a laminated glass structure, whereby it ispossible to prevent the glass from cracking and scattering.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described in detailon the basis of the accompanying drawings, wherein:

FIG. 1 is a partially sectional view showing the construction of a radiowave absorbing panel according to the invention;

FIG. 2 is a pattern view showing conducting films according to theinvention disposed in the form of stripes;

FIG. 3 is a pattern view showing conducting films according to theinvention disposed in the form of a matrix;

FIG. 4A is a construction view of an insulating substrate on the surfaceof which are formed stripe-form conducting films,

FIG. 4B is a plan view of the stripe-form conducting films, and

FIG. 4C is a sectional view on the line C—C in FIG. 4B;

FIG. 5 is a radio wave transmission, reflection and absorption frequencycharacteristic chart for a case where an insulating substrate on whichconducting films in the form of stripes are formed is not interposedbetween two facing insulating substrates;

FIG. 6 is a radio wave transmission, reflection and absorption frequencycharacteristic chart for a case where an insulating substrate on whichconducting films in the form of stripes are formed is interposed betweentwo insulating substrates according to the invention;

FIG. 7 is a graph showing theoretical calculation results obtained for afirst preferred embodiment of the invention, Preferred Embodiment 1; and

FIG. 8 is a graph showing theoretical calculation results obtained for afirst comparison example of the invention, Comparison Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention provides a thin radio wave absorbing panel whichcan be applied to window glass of a building and which both providestransparency to visible light and absorbs radio waves from outsideprevent the reflection of radio waves and the transmission of radiowaves into the building.

In FIG. 1, a radio wave absorbing panel 1 is made up of a firstinsulating substrate 2 forming one side of the panel, a first conductingfilm 3 coated on the entire surface of the inner side of the firstinsulating substrate 2, a second insulating substrate 6 disposed inparallel with and a predetermined distance away from the firstinsulating substrate 2 and forming the other side of the panel, a secondinsulating film 7 coated on the entire surface of the inner side of thesecond insulating substrate 6, a middle insulating substrate 4 disposedbetween and in parallel with the first insulating substrate 2 and thesecond insulating substrate 6, and a conducting film 5 coated on thesurface of the middle insulating substrate 4 in the form of stripes orin the form of a matrix. The insulating substrates 2, 6, 4 are fixedlysecured and air layers 8 are sealed.

Although in the preferred embodiment shown in FIG. 1 the radio waveabsorbing panel 1 has a three-layer structure made up of the firstinsulating substrate 2, the middle insulating substrate 4 and the secondinsulating substrate 6, the panel 1 may alternatively be made up of aplurality of such three-layer structures.

Also, the first conducting film 3 and the second insulating film 7 mayalternatively be coated continuously on the outer sides of therespective first and second insulating substrates 2 and 6, or may becoated continuously on both sides of the second insulating substrate 6.

Also, a plurality of middle insulating substrates 4 may be disposedbetween the first insulating substrate 2 and the second insulatingsubstrate 6.

Or, a single middle insulating substrate 4 and one or more ordinaryinsulating substrates may be disposed between the first insulatingsubstrate 2 and the second insulating substrate 6.

FIG. 2 shows a pattern of stripe-form conducting films of a radio waveabsorbing panel according to the invention.

As shown in FIG. 2, stripe-form conducting films 5A have a stripe widthH and a gap D between adjacent stripes.

FIG. 3 shows a pattern of matrix-form conducting films or a radio waveabsorbing panel according to the invention.

In FIG. 3, a matrix of conducting films 5B is made up of multiplerectangular film pieces arrayed in the form of a matrix. The width ofthe film pieces in their row and column directions are respectively Hand V, and the gap between pieces adjacent in the row and columndirections is D.

FIGS. 4A through 4C are views of an insulating substrate according tothe invention made by forming stripe-shaped conducting films on asubstrate.

FIG. 4A shows the construction of a middle insulating substrate 4 havingstripe-form conducting films 5A formed on its surface and alsoillustrates the action of a uniform radio wave field impressed on thestripe-form conducting films 5A. FIG. 4B shows the pattern of thestripe-form conducting films 5A. FIG. 4C is a sectional view of thestripe-form conducting films 5A shown in FIG. 4B on the line C—C.

In FIG. 4A, the y-axis direction vector shows the direction of theuniform radio wave field and the vector having the x-z coordinateinclination α shows the propagation direction of the electromagneticwaves.

In FIG. 4C, the major axis of the flat ellipse (the width of thestripe-form film) of each of the stripe-form conducting films 5A incross-section is H, the minor axis (the thickness of the film) is A, andthe gap between adjacent stripe-form films is D. The insulationresistance between adjacent stripe-shaped films will be written R_(D),and the relative permittivity of the surroundings of the flat ellipticalfilms will be written ε^(eX).

Generally, when two insulating substrates on an entire surface of eachof which a conducting film has been formed are disposed in parallel(i.e. when in FIG. 1 there is no middle insulating substrate 4 butrather just an air layer 8), to produce radio wave absorptionaccompanying resonance caused by interference of multiple reflections,it is necessary to make the distance between the two substrates about ¼of the wavelength of the radio waves. Therefore, because when the spacebetween the two substrates is an air layer its relative permittivity is1, with respect to radio waves of for example 100 MHz in frequency thedistance between the two substrates must be 75 cm. When glass, which hasa high relative permittivity, is used instead of air, because itsrelative permittivity is about 7, the distance between the twosubstrates becomes about 28 cm. To reduce the distance between the twosubstrates to about 10 cm, with respect to radio waves of frequency 100MHz, a medium having a relative permittivity of several tens or morebecomes necessary.

Research carried out by the present inventors has shown that theconducting films 5A divided up in the form of stripes shown in FIG. 4Chave the property that with respect to a field of radio wavesperpendicular to the centerline of a gap between adjacent stripe-formfilms, under certain conditions, if the film thickness of thestripe-form conducting films 5A is written Acm and the major axis of theflat ellipse of the cross-sections of the films is written Hcm, theyhave a huge real relative permittivity of about H/A (in a normal caseabout 10⁷), and also can be regarded as a continuous medium of thicknessAcm.

A radio wave absorbing panel according to the invention has the samefunction as when a dielectric film having this kind of huge realpermittivity is disposed between two insulating substrates disposed inparallel on each of which a continuous conducting film is formed, andcan effect a large phase change in radio waves across the conductingfilms divided up in the form of stripes or a matrix. As a result, evenwhen the distance between the two insulating substrates (the panelthickness) is much less than ¼ of a wavelength, it is possible toproduce resonance derived from interference of multiple reflections andthereby realize a high radio wave absorbency.

Next, an approximate calculation of the above-mentioned huge realpermittivity will be carried out on the basis of a simple model.

Modeling was carried out by taking a stripe-form conducting film 5A as aflat elliptical prism extending in a direction perpendicular to theorientation of a radio wave field (the y-axis direction) as shown inFIG. 4C, forming the gaps between the adjacent elliptical prisms as aninsulation resistance R_(D) and a distance (width) D, and surroundingthe elliptical prism with a dielectric of relative permittivity ε^(eX);the polarization of a case wherein a uniform outside field is impressedon this one elliptical prism was computed, and by space-averaging overthe whole layer of the divided insulating films the effective relativepermittivity Σ_(BM) of this virtual dielectric layer is given by Exp.(1). $\begin{matrix}\begin{matrix}{\sum\limits_{BM}{= \quad {1 + \frac{\begin{matrix}{{{\pi \left( {H/\lambda} \right)}\quad\left\lbrack \left\{ ɛ^{e\quad X}\kappa \right. \right.} +} \\{\left. {\left( {ɛ^{e\quad X} - 1} \right)\quad \left( {D \cdot {R_{BM}/R_{D}}} \right)} \right\} - \left. \left( {H \cdot {R_{BM}/R_{D}}} \right) \right\rbrack}\end{matrix}}{\left( {\pi \quad {A/\lambda}} \right)\quad \left\{ {1 + {\left( {H/\lambda} \right)^{2}\left( {2\quad \pi} \right)^{2}\left( {R_{BM}/Z_{O}} \right)^{2}}} \right\}} +}}} \\{\quad {i\frac{\begin{matrix}{{\left( {H + D} \right)\quad \left( {{Z_{O}/2}R_{D}} \right)} +} \\{2\quad \pi^{2}{ɛ^{eX}\left( {H/\lambda} \right)}^{2}\left( {\kappa + {D \cdot {R_{BM}/R_{D}}}} \right)\quad \left( {R_{BM}/Z_{O}} \right)}\end{matrix}}{\left( {\pi \quad {A/\lambda}} \right)\quad \left\{ {1 + {\left( {H/\lambda} \right)^{2}\left( {2\pi}\quad \right)^{2}\left( {R_{BM}/Z_{O}} \right)^{2}}} \right\}}}}\end{matrix} & (1)\end{matrix}$

where:

A is the minor axis of the elliptical prism (equivalent to theconducting film thickness);

H is the major axis of the elliptical prism (equivalent to the width ofthe stripe-form conducting films);

D is the width of the high-resistance part dividing adjacent stripe-formfilms;

k is the coverage, (π/4)AH/{A(H+D)};

λ is the wavelength [cm] in a vacuum of the incident radio waves;

Z₀ is the space impedance of a vacuum, 4πc/109≈377Ω;

R_(BM) is the sheet resistivity of the stripe-form film [Ω/□]; and

R_(D) is the insulation resistance between adjacent stripe-form films ofunit length [cmΩ].

When the insulation resistance R_(D) of the gaps between the stripe-formfilms is sufficiently large, by taking R_(D) as infinity, Exp. (1) canbe simplified to Exp. (2). $\begin{matrix}\begin{matrix}{\sum\limits_{BM}{= \quad {1 + \frac{ɛ^{e\quad X}\left( {H/A} \right)}{1 + {\left( {H/\lambda} \right)^{2}\left( {2\quad \pi} \right)^{2}\left( {R_{BM}/Z_{O}} \right)^{2}}} +}}} \\{\quad {2\quad \pi \quad i\frac{ɛ^{eX}{\kappa \left( {H/A} \right)}\quad \left( {H/\lambda} \right)\quad \left( {R_{BM}/Z_{O}} \right)^{2}}{1 + {\left( {H/\lambda} \right)^{2}\left( {2\pi}\quad \right)^{2}\left( {R_{BM}/Z_{O}} \right)^{2}}}}}\end{matrix} & (2)\end{matrix}$

Also, when the major axis H of the elliptical conducting films is set toa value sufficiently smaller than the wavelength λ of the incident radiowaves (H<<λ) and the sheet resistivity R_(BM) of the conducting films isset to a value sufficiently smaller than the space impedance of a vacuumZ₀ (R_(BM)<<Z₀), Exp. (2) can be simplified to Exp. (3).

Σ_(BM)⇄ε^(eX)k (H/A)  (3)

From Exp. (3), the effective relative permittivity Σ_(BM) of the virtualdielectric layer of the stripe-form conducting films 5A shown in FIGS.4A through 4C is equivalent to a huge real relative permittivity (about10⁷).

Using the relative permittivity Σ_(BM) shown in Exp. (3), the radio wavetransmission, reflection and absorption characteristics of when with twoinsulating substrates on each of which is formed a continuous conductingfilm disposed in parallel an insulating substrate on which is formed aconducting film divided up into stripes is interposed therebetween werecalculated using Maxwell's equations.

Calculated results of radio wave transmission, reflection and absorptionfrequency characteristics for when with two insulating substrates oneach of which is formed a continuous conducting film disposed inparallel an insulating substrate on which is formed a conducting filmdivided up into stripes is not and is interposed therebetween are shownin FIG. 5 and FIG. 6, respectively.

FIG. 5 shows radio wave transmission, reflection and absorptionfrequency characteristics of when an insulating substrate on which isformed a conducting film divided up into stripes is not provided. Inthis case, the thickness of the air layer between the two insulatingsubstrates on each of which a continuous conducting film is formed istaken as 60 cm.

In FIG. 5, in the vicinity of the frequency 100 MHz, the radio waveabsorptance is very high (absorptance=1), while the radio wavereflectivity decreases greatly (reflectivity=0.001; attenuation 60 dB).

FIG. 6 shows radio wave transmission, reflection and absorptionfrequency characteristics of when a middle insulating substrate on whichis formed a conducting film divided up into stripes according to theinvention is interposed between first and second insulating substrates.The gap between the first insulating substrate 2 and the middleinsulating substrate 4 shown in FIG. 1 was taken as 15 cm and the gapbetween the middle insulating substrate 4 and the second insulatingsubstrate 6 as 1 cm.

In FIG. 6, in the vicinity of the frequency 100 MHz, the radio waveabsorptance is very high (absorptance=1), while the radio wavereflectivity decreases greatly (reflectivity=0.01; attenuation 40 dB).

In this way, in a radio wave absorbing panel according to the invention,because a middle insulating substrate on which is formed a conductingfilm divided up into stripes is provided between at least two insulatingsubstrates on each of which a continuous conducting film is formed, evenwhen the panel width is reduced to ¼ compared to when there is no middleinsulating substrate on which is formed a conducting film divided upinto stripes, the radio wave reflectivity can be reduced and theabsorptance increased.

In the characteristics of the continuous conducting films used in theradio wave absorbing panel of the invention there is an action ofreflecting nearly 100% of radio waves and an action of reflecting ortransmitting some radio waves and absorbing some.

To obtain the former characteristic, a conducting film having a smallsheet resistivity R_(BM) is suitable, but to make the sheet resistivityR_(BM) small it is necessary to make the conducting film thick, and itsmanufacturing cost increases. Also, if a metal film is used as theconducting film, when the conducting film is made thick its transparencyto visible light decreases. For this reason, to eliminate problems ofmanufacturing cost and visible light transparency, the sheet resistivityR_(BM) is set between 1Ω/□ and 3000Ω/□. Most preferably, the sheetresistivity R_(BM) is set between 5Ω/□ and 20Ω/□ so that visible lighttransparency which is not much different from that of ordinarytransparent glass can be obtained.

In the case of the latter characteristic, when multiple insulatingsubstrates coated with continuous conducting films are used the radiowave absorption characteristics variously change, and when the sheetresistivity R_(BM) is made greater than 3000Ω/□ there is too muchtransmission of radio waves and it is difficult to obtain sufficientabsorption, and when on the other hand the sheet resistivity R_(BM) ismade less than 50Ω/□ it becomes difficult to obtain sufficientabsorption because there is too much radio wave reflection. Therefore,the sheet resistivity R_(BM) is set preferably between 200Ω/□ and1500Ω/□.

In the characteristics of the stripe-form or matrix-form conductingfilms, to raise the radio wave absorptivity the effective relativepermittivity Σ_(BM) must be made large. When the real part of therelative permittivity Σ_(BM) is not sufficiently large compared with theimaginary part, resonance does not readily occur and the absorptancefalls, and therefore it is important for the major axis H of theelliptical conducting films to be set to a value sufficiently smallerthan the wavelength λ of the incident radio waves (H<<λ) and for thesheet resistivity of the conducting films to be set sufficiently smallerthan the space impedance of a vacuum Z₀ (R_(BM)<<Z₀).

From Exp. (3), the relative permittivity Σ_(BM) can be set to a largevalue when the value of the width H of the stripe-form conducting filmsis made large, but because the wavelength λ of the arriving radio waveswhen they are in the VHF band is about 300 cm, to obtain H<<λ the widthH of the stripe-form conducting films is made smaller than 50 cm.

When the width H of each stripe-form conducting film is made small, therelative permittivity Σ_(BM) becomes small, whereby the radio waveabsorptance becomes small, and also the conducting films becomedifficult to manufacture. For this reason, the value of H is normallyset to at least 1 cm (H≧1 cm). Stated otherwise, the width H of eachstripe-form conducting film falls in the range of 1 cm≦H≦50 cm.

For the same reason as in the stripe-form conducting film, therow-direction pattern width V of each matrix-form conducting film shownin FIG. 3 is also in the range of 1 cm≦V≦50 cm.

The sheet resistivity R_(BM) of the stripe-form conducting films must asmentioned above be made a value sufficiently smaller than the spaceimpedance of a vacuum Z₀ (R_(BM)<<Z₀⇄377Ω), and normally the sheetresistivity R_(BM) is set to be not more than 40Ω/□ (R_(BM) ²40Ω/□).

However, to reduce the sheet resistivity R_(BM) it is necessary toincrease the thickness of the conducting films but then manufacturingcost increases. When the film thickness of the conducting films andmetal films are used as the conducting films, there arises the problemthat transmittivity of visible light decreases. For this reason thesheet resistivity R_(BM) is set to at least 1Ω/□ (R_(BM)≧1Ω/□) andpreferably from 5Ω/□ to 20Ω/□.

In the conditions for the real part of the relative permittivity Σ_(BM)being larger than the imaginary part, it is necessary for the insulationresistance R_(D) of the gap between adjacent stripe-form films to belarge, as shown in Exp. (4).

π(H/λ)ε^(eX)k>>(H+D) (Z₀/2R_(D))  (4)

Normally, the width H of the stripe-form conducting films is generallyset amply larger than adjacent the stripe-form film gap D (H>>D), andapplying this condition to Exp. (4) yields Exp. (5).

R_(D)>>2λZ₀/(π²ε^(eX))  (5)

When radio wave wavelength λ=300 cm, relative permittivity ε^(eX)=7(glass, and Z₀=377Ω are substituted into Exp. (5), insulation resistanceof stripe-form film gap R_(D)>>3300 cmΩ is obtained, and normally theinsulation resistance R_(D) is set greater than 30,000 cmΩ.

When on a conducting film divided up in one direction in the form ofstripes a field of radio waves is impressed in the length direction ofthe stripes, the effect of the stripe-form conducting films discussedabove cannot be expected, and the same characteristics as those of acontinuous conducting film are obtained.

To obtain the same radio wave absorbing characteristics as stripe-formconducting films with respect to an electric field of any orientation,the conducting films are set to a matrix form. In the matrix shape, theconditions of the unidirectional stripe shape mentioned above must besatisfied.

Next, as a preferred embodiment of a radio wave absorbing panelaccording to the invention, a panel made by forming on one of theinsulating substrates constituting the panel shown in FIG. 1 aconducting film having an action of reflecting nearly 100% of radiowaves and forming on the other of the insulating substrates a conductingfilm having the action of partially reflecting and transmitting andpartially absorbing radio waves and disposing between these twosubstrates at least one insulating substrate on a surface of which areformed matrix-form conducting films will be described.

In this kind of radio wave absorbing panel, compared with a case whereinan insulating substrate having the function of partially reflecting andtransmitting and partially absorbing radio waves is disposed over theentire face of the outer side of the panel, the thickness of the airlayer can be reduced to about ½, and also the effect of the insulatingsubstrate on which the stripe-form or matrix-form conducting films areformed can be made the equivalent of the effect of double the number ofsuch substrates.

The sheet resistivity R_(BM) of the continuous conducting film havingthe function of reflecting nearly 100% of radio waves is set to 1Ω/□ to30Ω/□. A particularly optimal sheet resistivity R_(BM) is 5Ω/□ to 20Ω/□.However, when transparency to light is not required, a conducting filmhaving a lower sheet resistivity R_(BM) can be used and a more markedeffect can thereby be obtained.

The sheet resistivity R_(BM) of the continuous conducting film havingthe function of partially reflecting and transmitting and partiallyabsorbing radio waves is set to 50Ω/□ to 3000Ω/□. A particularly optimalsheet resistivity R_(BM) is 200Ω/□ to 1500Ω/□.

To apply a radio wave absorbing panel according to the invention to awindow opening of a building it is necessary to use substrates which aretransparent to visible light as the insulating substrates, andtransparent plate glass is suitable. Also for the conducting filmsformed on the insulating substrates, conducting films transparent tolight are used. In particular, to make the sheet resistivity of thestripe-form or matrix-form conducting films low and also maintaintransparency to light, the film material must be selected from a limitedrange of alternatives. As film materials with which this kind offunction can be realized, for example transparent conducting filmscomposed mainly of SnO₂ or In₂O₃ or metal films composed mainly of Ag,Au, Cu and Al are suitable.

Because these conducting films or metal films reflect near-infrared raysof sunlight and have a low heat ray radiation function, in addition totheir radio wave absorption function, when the panel is used as windowglass, they enable energy of room air-conditioning to be saved.

When these film materials, and particularly metal films mainly composedof either Ag, Au, Cu or Al are used, to ensure durability of the films,multiple layer glass made by sealing dry air between glass substrates,or laminated glass made by inserting resin between glass substrates, ispreferably used. To impart a radio wave absorbing action using dry airor resin, the thicknesses of the air layers, the thicknesses of theglass substrates and the thicknesses of the resin layers are set on thebasis of the conditions discussed above.

Preferred embodiments of the invention will be described specificallyalong with comparison examples.

First Preferred Embodiment

Using an in-line sputtering apparatus, a gas of 90 mol % nitrogen, 10mol % oxygen was introduced and by reactive sputtering using a Cr metaltarget an oxide nitride CrOxNy film was formed on a sheet of soda limesilica glass of thickness 4 mm. The thickness of this film was about 40nm and the sheet resistivity of the film was about 400Ω/□.

Next, by means of an in-line sputtering apparatus, on a soda lime silicaglass substrate of thickness 4 mm, films of ZnO, Ag and ZnO were formedin order from the substrate side as an Ag film. The in-line sputteringapparatus was controlled so that the respective film thicknesses were 40nm, 15 nm and 40 nm. The ZnO films have the role of maintaining thedurability of the Ag film. The sheet resistivity of the film obtainedwas about 5Ω/□.

Also, using the same in-line sputtering apparatus, a thin stainlesssteel plate mask of thickness about 0.5 mm having square holes of sidelength 5 cm formed therein with gaps of 4 mm between adjacent holes wasprepared and placed on a soda lime silica glass substrate of thickness 4mm, and films of ZnO, Ag and ZnO were formed in order from the glasssubstrate side with the in-line sputtering apparatus being controlled sothat the film thicknesses were respectively 40 nm, 15 nm and 40 nm.

The mask was then removed to form 5 cm-square transparent ZnO/Ag/ZnOfilms on the substrate in the form of a matrix. The sheet resistivity ofthe films was about 5Ω/□, and the insulation resistance between adjacentfilms was over 20MΩ.

These three glass substrates were then cut into squares of side 120 cmand used to construct a laminate of the structure: 4 mm glass/CrOxNy:0.6cm air layer:matrix of ZnO/Ag/ZnO films/4 mm glass:2.4 cm airlayer:ZnO/Ag/ZnO film/4 mm glass.

When the radio wave reflectivity and the radio wave transmittivity from200 MHz to 1 GHz of this laminate were measured, it was found that ataround 500 MHz the reflectivity decreased and there was a region wherestrong resonant absorption occurred.

Computed results of radio wave reflectivity and absorptioncharacteristics obtained using the approximate theory discussed abovefor this panel are shown in FIG. 7.

First Comparison Example

A laminate was made by replacing the glass substrate of thickness 4 mmhaving a matrix-form Ag film formed thereon of the first preferredembodiment with a glass substrate having no film formed on it, and theradio wave reflectivity and the radio wave transmittivity from 200 MHzto 1 GHz of this laminate were measured. The result was that althoughthere was a decrease in reflectivity and an increase in transmittivityat around 1 GHz, at below 1 GHz there was no resonant absorption.

In theoretically computed results of radio wave reflectivity andtransmittivity obtained for this construction, as shown in FIG. 8 therewas resonant absorption at around 1.2 GHz.

To induce resonant absorption at around the same frequency of 500 MHz asthe first preferred embodiment, it was necessary to increase thethickness of the 2.5 cm air layer to 10 cm.

Second Preferred Embodiment

An indium tin oxide (ITO) sintered target was mounted in an in-linesputtering apparatus and a gas made by adding 20 mol % oxygen to Ar wasintroduced to form an ITO film of thickness about 60 nm on a soda limesilica glass substrate of thickness 4 mm. The film-forming conditionswere controlled so as to bring the sheet resistivity of the film toabout 400Ω/□.

A laminate was made by substituting this substrate for the substratehaving the CrOxNy film formed thereon of the first preferred embodimentand the radio wave reflectivity and transmittivity from 200 MHz to 1 GHzof this laminate were measured. The result was that the laminateexhibited substantially the same characteristics as the first preferredembodiment.

Third Preferred Embodiment

Using an in-line sputtering apparatus, SnO₂, Ag and SnO₂ films wereformed on a soda lime silica glass substrate of thickness 4 mm. The filmthicknesses were respectively controlled to 40 nm, 15 nm and 40 nm, anda film of sheet resistivity 5Ω/□ was obtained.

The film face of this glass substrate was scored in a checkerboardpattern using a steel needle and thereby divided up into squares of sidelength about 20 cm. When the divisions were viewed with an opticalmicroscope the film was seen to have been cut away in lines of justunder 200 μm in width. The electrical resistance between adjacentconducting films thus divided was generally over about 50kΩ. A laminatewas made by substituting this glass substrate with films in the form ofa matrix of 20 cm squares formed thereon for the glass substrate havingthe matrix of ZnO/Ag/ZnO films formed thereon of the first preferredembodiment, and the radio wave reflectivity and transmittivity from 200MHz to 1 GHz of this laminate were measured. The result was that aslightly weaker resonant absorption than that in the first preferredembodiment occurred at around 250 MHz.

Second Comparison Example

Using an in-line sputtering apparatus, ITO, Ag and ITO films were formedon a glass substrate. The film thicknesses were respectively controlledto 40 nm, 15 nm and 40 nm, and a film of sheet resistivity about 5Ω/□was obtained.

In the same way as in the third preferred embodiment, the film face wasscored in a checkerboard pattern using a steel needle and therebydivided up into squares of side length about 20 cm. The electricalresistance between adjacent conducting films thus divided was less than50Ω (about 1000 cmΩ as R_(D)).

A laminate was made by substituting this glass substrate with films inthe form of a matrix of 20 cm squares formed thereon for the glasssubstrate having the matrix of ZnO/Ag/ZnO films formed thereon of thefirst preferred embodiment, and the radio wave reflectivity andtransmittivity from 200 MHz to 1 GHz of this laminate were measured. Theresult was that although there was a gentle decrease of reflectivityaround frequency 250 MHz, resonant absorption did not occur.

Fourth Preferred Embodiment

The Cr oxide nitride film CrNxOy of sheet resistivity about 400Ω/□mentioned in the first preferred embodiment was formed on glass ofthickness 10 mm and this was cut into a square of side length 80 cm andstood perpendicular to a floor surface.

Cubes of foam styrol of side length 1 cm were then affixed to the filmface side of this glass as spacers at lattice points about 30 cm apart.

Colorless float glass of plate thickness 4 mm coated with a transparentconducting film consisting of a SnO₂ film of sheet resistivity about10Ω/□ doped with fluorine of film thickness about 300 nm was cut intostrips of width 20 cm, length 180 cm, and these were lined up closely inthe longitudinal direction on a plate of plastic and fixed to make apanel of side length 180 cm.

This panel was stood vertical and placed against the 10 mm-thick glasswith the foam styrol spacers therebetween.

Also, so as to sandwich this panel and parallel therewith another panelhaving aluminum foil of thickness 15 μm stretched over it was sodisposed that the thickness of an air layer formed between the panelscould be varied.

Linearly polarized radio waves were then directed at the laminate thusconstructed from the 10 mm-thick glass side, and its reflectivity in afrequency range of 50 MHz to 500 MHz was measured.

When the polarization of the radio wave field was horizontal and thethickness of the variable air layer was about 15 cm, at about 100 MHzthere was a marked fall in reflectivity and strong resonant absorptionoccurred.

Third Comparison Example

When radio waves whose field was polarized vertically were directed atthe laminate construction of the fourth preferred embodiment, thereflectivity in the frequency range of 50 MHz to 500 MHz wassubstantially 100%.

Fourth Comparison Example

From the laminate construction of the fourth preferred embodiment, thepanel made by lining up 20 cm×180 cm pieces of glass coated withtransparent conducting film was removed and the thickness of the airlayer was varied to investigate the condition under which resonantabsorption with respect to horizontally polarized waves arose at around100 MHz. It was found that a thickness of the air layer of about 70 cmwas necessary.

Fifth Preferred Embodiment

A soda lime silica glass substrate of thickness 4 mm coated with acontinuous ITO film of sheet resistivity about 400Ω/□, a soda limesilica glass substrate of thickness 18 mm coated with transparentconducting films made by forming a SnO₂/Ag/SnO₂ film of sheetresistivity about 5Ω/□ and then dividing this up into a matrix ofsquares of side length 5 cm by scoring with a steel needle, and a sodalime silica glass substrate of thickness 4 mm coated with a continuousZnO/Ag/ZnO/Ag/AnO film of sheet resistivity about 3Ω/□ were laminatedtogether with resin interposed therebetween to form a laminate panel ofthe construction: 4 mm glass/ITO film:0.36 mm resin:divided SnO₂/Ag/SnO₂film/18 mm glass:0.36 mm resin:ZnO/Ag/ZnO/Ag/ZnO film/4 mm glass.

Radio waves were directed at this laminate panel from the side of theglass substrate coated with the ITO film, and its radio wavereflectivity and radio wave transmittivity from 400 MHz to 1.5 GHz weremeasured. It was found that resonant absorption occurred at around 600MHz.

Fifth Comparison Example

A laminate glass was constructed by using a soda lime silica glasssubstrate of thickness 18 mm not coated with anything in place of thesoda lime silica glass substrate of thickness 18 mm on which the matrixof transparent conducting films was formed in the laminate constructionof the fifth preferred embodiment.

When radio waves were directed at this laminate panel from the side ofthe glass substrate coated with the ITO film and the radio wavereflectivity and radio wave transmittivity from 400 MHz to 1.5 GHz weremeasured, resonant absorption occurred at around 1.2 GHz.

Industrial Applicability

As described above, with this invention it is possible to increase therelative permittivity of a radio wave absorbing panel and obtain a thinradio wave absorbing panel having excellent radio wave absorptance aswell as excellent transparency to light. Thus, a radio wave absorbingpanel according to the invention is ideal as a panel for installation inwindow openings of buildings.

What is claimed is:
 1. A radio wave absorbing panel comprising: at leasttwo insulating substrates disposed in parallel a predetermined distanceapart on at least one side of each of which is formed a continuousconducting film; and at least one insulating substrate disposed betweenthese insulating substrates in parallel therewith on a surface of whichare formed conducting films disposed in the form of stripes or in theform of a matrix.
 2. A radio wave absorbing panel according to claim 1,wherein the sheet resistivity of the continuous conducting film is inthe range of 1Ω/□ to 3000Ω/□, and when the width and the sheetresistivity of each of the conducting films disposed in the form ofstripes are respectively written Hcm and R_(BM)Ω/□ and the insulationresistance of the insulating substrate on which the conducting filmsdisposed in the form of stripes are formed is written R_(D)cmΩ, then H,R_(BM) and R_(D) have the following values: 1 cm≦H≦50 cm1Ω/□≦R_(BM)≦40Ω/□ R_(D)≧30,000 cmΩ.
 3. A radio wave absorbing panelaccording to claim 1, wherein the sheet resistivity of the continuousconducting film is in the range of 1 Ω/□ to 3000Ω/□, and when the width,the length and the sheet resistivity of each of the conducting filmsdisposed in the form of a matrix are respectively written Hcm, Vcm andR_(BM)Ω/□ and the insulation resistance of the insulating substrate onwhich the conducting films disposed in the form of a matrix are formedis written R_(D)cm, then H, V, R_(BM) and R_(D) have the followingvalues: 1 cm≦H≦50 cm 1 cm≦V≦50 cm 1Ω/□≦R_(BM)≦40Ω/□ R_(D)≧30,000 cmΩ. 4.A radio wave absorbing panel according to claim 1, wherein the sheetresistivity of the continuous conducting film formed on the surface ofone of the insulating substrates is in the range of 1Ω/□ to 30Ω/□ andthe sheet resistivity of the continuous conducting film formed on thesurface of another of the insulating substrates is in the range of 50Ω/□to 3000Ω/□, and when the width and the sheet resistivity of each of theconducting films disposed in the form of stripes are respectivelywritten Hcm and R_(BM)Ω/□ and the insulation resistance of theinsulating substrate on which the conducting films disposed in the formof stripes are formed is written R_(D)cmΩ, then H, R_(BM) and R_(D) havethe following values: 1 cm≦H≦50 cm 1Ω/□≦R_(BM)≦40Ω/□ R_(D)≧30,000 cmΩ.5. A radio wave absorbing panel according to claim 1, wherein the sheetresistivity of the continuous conducting film formed on the surface ofone of the insulating substrates is in the range of 1Ω/□ to 30Ω/□ andthe sheet resistivity of the continuous conducting film formed on thesurface of another of the insulating substrates is in the range of 50Ω/□to 3000Ω/□, and when the width, the length and the sheet resistivity ofeach of the conducting films disposed in the form of a matrix arerespectively written Hcm, Vcm and R_(BM)Ω/□ and the insulationresistance of the insulating substrate on which the conducting filmsdisposed in the form of a matrix are formed is written R_(D)cmΩ, then H,V, R_(BM) and R_(D) have the following values: 1 cm≦H≦50 cm 1 cm≦V≦50 cm1Ω/□≦R_(BM)≦40Ω/□ R_(D)≧30,000 cmΩ.
 6. A radio wave absorbing panelaccording to claim 1, wherein transparent plate glass is used as theinsulating substrates and the stripe or matrix form conducting films aretransparent conducting films composed mainly of SnO₂ or In₂O₃ or aremetal films composed mainly of Ag, Au, Cu or Al.
 7. A radio waveabsorbing panel according to claim 1, wherein dry air is sealed inspaces between the at least two insulating substrates and the at leastone insulating substrate on the surface of which are formed conductingfilms disposed in the form of stripes or a matrix.
 8. A radio waveabsorbing panel according to claim 1, wherein resin is sealed in spacesbetween the at least two insulating substrates and the insulatingsubstrate on the surface of which are formed conducting films disposedin the form of stripes or a matrix to form a laminated glass structure.